All publications and patent applications herein are incorporated by reference for all purposes to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.
The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventions, or that any publication specifically or implicitly referenced is prior art.
The goal of plant breeding is to combine in a single variety or hybrid various desirable traits, or to provide a desirable trait without significant detriment to other important properties. For field crops, desirable traits may include resistance to diseases and insects, tolerance to heat, cold and drought, reducing the time to crop maturity, greater yield, and better agronomic quality. With mechanical harvesting of many crops, uniformity of plant characteristics such as germination and stand establishment, growth rate, maturity, and plant and ear height is important. Other desirable traits may be those directly or indirectly associated with special nutritional and industrial types of crops. Examples of such specialty varieties or hybrids include those with higher oil content, different oil profiles, greater protein content, better protein quality or higher amylose content. It is also desirable to produce plants which are particularly adapted to a given agricultural region. New hybrids are an important part of efforts to control raw material costs.
Maize (Zea mays L.) is often referred to as corn in the United States, and the terms are used interchangeably in the present application. Maize has separate male and female flowers on the same plant, located on the tassel and the ear, respectively. Thus, it can be bred by crossing to itself (self-pollination or selfing), to another plant of the same family, line or variety (sib-pollination or sib-crossing) or to another plant of a different family, line or variety (outcrossing or cross-pollination).
Repeated self-pollination of plants, combined with selection for the desired type over many generations, results in inbred lines which are homozygous at almost all loci and thus will produce a uniform population of homozygous offspring when subject to further self-pollination. A cross between two different homozygous lines produces a uniform population of heterozygous hybrid plants. A cross of two plants each heterozygous at a number of gene loci will produce a population of heterogeneous plants that differ genetically and will not be uniform.
Hybrid maize varieties can be produced by a process comprising (1) the selection of plants from various germplasm pools for initial breeding crosses; (2) the selfing of the selected plants from the breeding crosses for several generations to produce a series of inbred lines as described above; and (3) crossing a selected inbred line with a different inbred line to produce the hybrid progeny (F1). Preferably, an inbred line should comprise homozygous alleles at about 95% or more of its loci.
Pedigree breeding and recurrent selection are two examples of methods used to develop an inbred line.
Pedigree breeding starts with the crossing of two or more genotypes, each of which may have one or more desirable characteristics. Superior progeny are selfed and selected in successive generations, during the course of which the level of homozygosity is increased. An inbred line suitable for hybrid production may be produced after a number of generations of selfing and selection, for example after four, five, six or more generations.
Double haploid methods can reduce the number of generations needed to obtain an inbred line. These methods involve the doubling of haploids derived from either the maternal or paternal gametes. Genetics markers can be used to identify haploids, and the haploids doubled to form homozygous diploid lines.
Recurrent selection entails individual plants cross-pollinating with each other to form progeny which are then grown. The superior progeny are then selected by any number of methods, which include individual plant, half sib progeny, full sib progeny, selfed progeny and topcrossing. The selected progeny are cross pollinated with each other to form progeny for another population. This population is planted and again superior plants are selected to cross pollinate with each other. The objective of this repeated process is to improve the traits of a population. The improved population can then be used as a source of breeding material to obtain inbred lines to be used in hybrids.
Backcrossing can be used to improve inbred lines and a hybrid which is made using those inbreds. Backcrossing can be used to transfer a specific desirable trait from one line, the donor parent, to an inbred called the recurrent parent which has overall good agronomic characteristics yet that lacks the desirable trait. This transfer can be achieved by first crossing the recurrent parent with the donor parent, and then performing a backcross in which the progeny are mated to the recurrent parent. The resultant progeny can then be selected for the desired trait, and a further backcross performed using the selected individuals. Typically after four or more backcross generations with selection for the desired trait in each generation, the progeny will contain essentially all genes of the recurrent parent except for the genes controlling the desired trait. The last backcross generation is then selfed to give pure breeding progeny for the gene(s) being transferred.
Other plant breeding techniques known in the art, such as restriction fragment length polymorphism enhanced selection, genetic marker enhanced selection and transformation, may also be used in the production of inbred lines. For example, selection in the breeding process can be based upon the accumulation of markers linked to the positive effecting alleles and/or the elimination of markers linked to the negative effecting alleles from the plant's genome. Often, a combination of techniques is used.
For a review of plant breeding methods well known to those skilled in the art, see, for example, Sprague and Dudley (eds.), Corn and Corn Improvement, Third Edition, American Society of Agronomy, Inc., 986 pages, 1988; Fehr and Hadley (eds.), Hybridization of Crop Plants, American Society of Agronomy, Inc., 765 pages, 1980; Allard, Principles of Plant Breeding, John Wiley & Sons, Inc., 485 pages, 1960; Jensen, Plant Breeding Methodology, John Wiley & Sons, Inc., 676 pages, 1988; Simmonds, Principles of Plant Breeding, Longman Group Limited, 408 pages, 1979; and Hallauer and Miranda, Quantitative Genetics in Maize Breeding, Iowa State University Press, 468 pages, 1981.
In producing a hybrid strain by crossing two different inbred lines, it is advantageous to minimize the possibility of self-pollination. Minimizing self-pollination will minimize the proportion of the resultant seed which is substantially identical to the inbred line (resulting from the self-pollination) and increase the amount of hybrid seed (resulting from cross pollination). To this end, commercial maize hybrid production uses a male sterility system to render the female parent male sterile. There are several ways in which a maize plant can be manipulated so that it is male sterile. These include use of manual or mechanical emasculation (or detasseling), cytoplasmic genetic male sterility, nuclear genetic male sterility or gametocides (chemical agents affecting cells critical to male fertility, for example as described in Carlson, Glenn R., U.S. Pat. No. 4,936,904).
In detasseling, alternate strips of two inbred varieties of maize are planted in a field, and the pollen-bearing tassels are removed from one of the inbreds (female) prior to pollen shed. Providing that there is sufficient isolation from sources of foreign maize pollen, the ears of the detasseled inbred will be fertilized only from the other inbred (male), and the resulting seed is therefore hybrid and will form hybrid plants.
Alternatively, the female line can be cytoplasmic male sterile as a result of an inherited factor in the cytoplasmic genome. This characteristic is inherited exclusively through the female parent in maize plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile. The same hybrid seed, a portion produced from detasseled fertile maize and a portion produced using the CMS system can be blended to insure that adequate pollen loads are available for fertilization when the hybrid plants are grown.
Genetic male sterility may be conferred by one of several available methods, such as multiple mutant genes at separate locations within the genome that confer male sterility, as disclosed in U.S. Pat. Nos. 4,654,465 and 4,727,219 to Brar et al. and chromosomal translocations as described by Patterson in U.S. Pat. Nos. 3,861,709 and 3,710,511. A system in which male fertility genes are expressed under an inducible promoter is described in Albertsen et al., U.S. Pat. No. 5,432,068. Other approaches include delivering into the plant a gene encoding a cytotoxic substance associated with a male tissue specific promoter, or an antisense system in which a gene critical to fertility is identified and an antisense to that gene is inserted in the plant (see Fabinjanski, et al. EPO 89/3010153.8 publication no. 329,308 and PCT application PCT/CA90/00037 published as WO 90/08828).
Having obtained a desirable hybrid strain by the crossing of two different parent inbred strains, it is possible to maintain a uniform supply of the hybrid seed. The population of parent plants can be maintained by repeated self pollination. Moreover, since the parents are homozygous, the hybrid produced in the cross will always be the same. Thus, once a desirable hybrid has been identified, a continual supply of hybrid seed having the same properties can be provided.
Objectives of commercial maize hybrid line development include the development of new corn hybrids which are able to produce high yield of grain, which require less investment of time or resources, which are more resistant to environmental stresses (e.g., stresses particular to a certain growing area), which are easier to harvest and/or which provide grain or other products particularly suitable for a desired commercial purpose. To obtain a new hybrid, the corn breeder selects and develops superior inbred parental lines for producing hybrids. This is far from straightforward in view of the number of segregating genes and in view of the fact that the breeder often does not know the desired parental genotype in detail. Then, the breeder must identify the particular cross-combination of inbred lines which produces a desired hybrid. Even having obtained two superior inbred lines, there is no guarantee that the combination of these will produce desirable hybrid F1 plants. This is particularly the case because many selectable traits (e.g., yield) are dependent on the effects of numerous genes interacting with each other. Thus, the selection or combination of two parent lines produces a unique hybrid which differs from that obtained when either of the parents is crossed with a different inbred parent line.